The somatomedin carrier protein isolated from BRL-3A-conditioned medium is a single chain polypeptide of about 32 kilodaltons. Cross-linking experiments have determined that the carrier protein forms a complex with MSA that consists of one molecule of carrier protein and one molecule of MSA.This complex has a Mr = 42 kilodaltons and is similar in size to the small somatomedin complex observed in animal sera. Antibody to the carrier protein subunit will be used to isolate both the small and large complexes intact from rat serum. Specific aims for the next year include the characterization of the composition of both serum complexes and the identification of the growth hormone-dependent, acid-sensitive component of the large endogenous complex. Identification of the carrier protein subunit and the somatomedin-binding subunit will be performed by labeled antibody and MSA binding following electrophoretic blotting of proteins from polyacrylamide gels. Analysis of conditions required for release of somatomedin from the complexes will be performed using complexes reconstructed from purified components. In addition, MSA binding to A1684 chondrosarcoma cells has been extensively characterized and correlated to biological activity. The MSA receptor has been identified by cross-linking techniques as a single chain polypeptide of 220 kilodaltons; a typical type II insulin-insensitive receptor. Further biochemical characterization of the receptor is planned following purification using affinity chromatography. (J)